Enforce uniqueness the way a UNIQUE constraint actually works

customers.email currently happens to hold unique values, but nothing in the schema enforces it — a duplicate could be inserted tomorrow. Enforce it by creating a UNIQUE index directly on customers.email, rather than going through ADD CONSTRAINT ... UNIQUE.

(They end up equivalent — a UNIQUE constraint is implemented as a unique index under the hood — but writing the index form directly is worth practicing, since it's the only form available when the unique rule needs to be partial or expression-based, which a plain constraint can't do.)